The present invention relates to a valve mechanism for an automotive engine and more particularly to a valve mechanism where a valve lift and timing of the valves are varied in accordance with various operating conditions.
In a four-cycle automotive engine, the valve timing and valve lift of the intake and exhaust valve are controlled in accordance with the engine operating conditions, so that an opening angle of the intake and exhaust valves and a valve overlap of both valves are changed.
In such a system, when the valve opening angle or the valve overlap is small, the performance of the engine in a low and middle engine speed range is improved.
On the other hand, it is desirable to quickly activate a catalyst in a catalytic converter and an O.sub.2 -sensor of an exhaust system by increasing the temperature of exhaust gases in order to improve emission control during the cold engine and idling state. To the contrary, it is preferable to cool the exhaust gases when an automobile is running except when increasing the engine torque in order to improve the durability of the exhaust system and to avoid trouble of the exhaust system caused by heat.
The temperature of the exhaust gas depends largely on the opening timing of the exhaust valve, that is, on the expansion ratio. Therefore, the emission control and the durability of the exhaust system can be improved by controlling the valve timing.
Japanese Patent Application Laid-Open 60-27711 discloses a system for controlling the valve mechanism wherein the opening timing of the exhaust valve is retarded in a middle engine load range so as to increase the expansion ratio, thereby improving fuel consumption. On the other hand, in a heavy engine load range, the valve opening timing of the exhaust valve is advanced to improve the exhaust efficiency, and to increase the power of the engine. Namely, the conventional control system controls the valve timing in the middle and heavy load range. Therefore, the system can not improve the emission in the cold engine. Since the exhaust valve is opened at an earlier timing in the heavy engine load range, the exhaust gas temperature increases, which causes trouble by heat, and reduces the durability of the valve mechanism.